The MAC hash key might be passed to us on stack. Copy it to
a slab buffer before mapping to gurantee proper DMA mapping.
Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>